The problems associated with the lubrication of gears such as utilized in automotive transmission and axles are well known to those skilled in the art. In the lubrication of automatic transmissions, proper fluid viscosity at both low and high temperatures is essential to successful operation. Good low temperature fluidity eases cold weather starting and insures that the hydraulic control system will properly "shift gears". High viscosity at elevated temperatures insures pumpability and the satisfactory operation of converters, valves, clutches, gears and bearings. These conflicting fluidity requirements require a product that exhibits the following characteristics:
(a) high temperature viscosity retention, PA1 (b) low temperature fluidity, PA1 (c) shear stability, and PA1 (d) high temperature stability.
In order to prepare lubricants having these characteristics, it has become common practice to add a variety of chemicals to the lubricating oil. For example, in order to meet the viscosity requirements, compositions have been added to the oils which are characterized by relatively small change in their viscosity with changing temperature. In general, lubricants containing such compositions have the desirable properties of functioning immediately, even though cold, upon being put into service, and to continue to function satisfactorily as they become heated during operation. Commonly used gear oil viscosity improvers include polymethacrylates and polyolefins.
In addition to viscosity improvers, lubricating compositions useful as gear lubricants generally will contain pour point depressants, extreme pressure agents, oxidation inhibitors, corrosion inhibitors, foam inhibitors, and friction modifiers.
Lubricating compositions have been suggested containing various nitrogen-containing and phosphorus-containing compositions to impart desirable properties to lubricating compositions. for example, U.S. Pat. No. 3,513,093 describes lubricant compositions containing substituted polyamines which comprise the reaction product of an alkylene amine with a substantially hydrocarbon-substituted succinic acid and at least about 0.001 mole of a phosphorus acid-producing compound selected from the group consisting of phosphoric acids, phosphorous acids, phosphonyl acids, phosphinyl acids, and the esters, the halides and the anhydrides thereof. The phosphorus acids may contain one or more sulfur atoms attached to the phosphorus atom. The substituted polyamines are useful as anti-wear agents, anti-rust agents, detergents, etc. U.S. Pat. No. 4,338,205 describes a lubricating oil with improved diesel dispersancy. The lubricating oils contain an acid-treated, oil-soluble alkenyl succinimide or a borated alkenyl succinimide which has been treated at an elevated temperature with an oil-soluble strong acid such as an alkyl sulfonic acid, or a phosphoric acid. The oil-soluble organic acids are generally classified as those acids containing a hydrogen-phosphorus moiety which has a pK of -10 to about +5.0.
The preparation of salts, both metal salts and ammonium salts of dialkylmonothiophosphoric acids has been described in the art. For example, U.S. Pat. No. 2,647,140 describes a process for preparing dialkylthiophosphates by reacting a dialkylphosphite with sulfur and ammonia or an organic amine. The product is an ammonium salt of a dialkylthiophosphate.
The preparation of O-alkyl-O-cycloalkyl-S-alkyl thiophosphoric acid esters is described in U.S. Pat. No. 3,294,874. The process involves initially forming ammonium salts of O,O-dialkylthiophosphoric acids, and it is suggested that the product characterized by the following formulae which are in equilibrium EQU R(O).sub.2 P(S)ONH.sub.4 .revreaction.R(0).sub.2 P(0)SNH.sub.4
the ammonium salts then are alkylated to form the desired S-alkylthiophosphoric acid esters.
The use of amine and metal salts of dialkylmonothiophosphoric acids in lubricating compositions also is described in the art such as in the following U.S. Pat. Nos.: 2,063,629; 2,224,695; 2,447,288; 2,616,905; 3,984,448; 4,431,552. Some of the above patents differ in the description of the alkyl groups and/or the nature of the metal or amine. For example, in U.S. Pat. No. 3,984,448, the alkyl groups contain from 1 to 6 carbon atoms, and in U.S. Pat. No. 2,447,288, the alkyl groups contain at least 5 carbon atoms. In U.S. Pat. No. 2,447,288, the amine used to form the amine salt is an aliphatic primary amine containing at least 8 carbon atoms, and in U.S. Pat. No. 2,063,629, the amine may be ammonia or a primary, secondary or tertiary amine.
Another publication which discusses the preparation of salts of dialkylthiophosphoric acids is Pesin, V. G. and Khaletskii, A. M., Zhurnal Obshchei Khimii, 31, No. 8, pp. 2508-2515, August, 1961. It is therein suggested that the salts of dialkylthiophosphoric acids are useful in the synthesis of insecticides, fungicides, bactericides, medicinal products, etc.
More recently, new demands are being placed on lubricants to be used in gear applications. Increases in commercial vehicle power and loading require the lubricant to be available to withstand severe thermal stressing while protecting the equipment being lubricated. Thus, the high temperature stability (e.g., above about 160.degree. C.) of lubricants designed for gear applications is a significant consideration.